1. Field of the Invention
The present invention generally relates to a common-rail injection system such as a high pressure fuel manifold or a block rail, etc. in an accumulating pressure fuel ejecting system of a diesel internal combustion engine, and particularly relates to a common-rail injection system for a diesel engine raised in internal pressure fatigue strength.
2. Description of the Prior Arts
As the common-rail injection system of this kind, for example, a common-rail injection system shown in FIG. 1, a common-rail injection system shown in FIG. 2, common-rail injection systems shown in FIGS. 3 and 4, an unillustrated block rail type common-rail injection system, etc. are conventionally known. In the common-rail injection system shown in FIG. 1, a boss 3c integrated with a main pipe rail 1 of the common-rail injection system is formed in this main pipe rail 1. A pressing seating face 2-3 formed in a connecting head portion 2-2 of a branch pipe 2 is abutted on a pressure receiving seating face 1-3 on the main pipe rail 1 side and is engaged with this pressure receiving seating face 1-3, and is connected by fastening a box nut 6 screwed onto a screw portion 3-2 arranged on the outer circumferential face of the above boss 3c. In the common-rail injection system shown in FIG. 2, a portion of a branch hole 1-2 communicated with an internal flow passage 1-1 of a circular section arranged in a circumferential wall portion on the side of a main pipe rail 1 is formed as a pressure receiving seating face 1-3 opened outward. A pressing seating face 2-3 formed in a connecting head portion 2-2 on the side of a branch pipe 2 as a branch connecting body formed in e.g., a tapering-off conical shape and enlarged in diameter by buckling molding in an end portion is abutted on the pressure receiving seating face 1-3 and is engaged with this pressure receiving seating face 1-3 by using a joint fitting 3 of a ring shape surrounding the outer circumferential portion of the main pipe rail 1 near the pressure receiving seating face. A portion of a screw wall 3-1 is projected outward from the main pipe rail 1, and is arranged in the joint fitting so as to be projected in the diametrical direction of the above main pipe rail 1. A nut 4 is assembled into the side of the branch pipe 2 in advance through a sleeve washer 5. The pressing seating face 2-3 is fastened and connected to the pressure receiving seating face 1-3 by pressing below a neck portion of the above connecting head portion 2-2 by screwing the nut 4 into the screw wall 3-1 portion. In the common-rail injection systems shown in FIGS. 3 and 4, sleeve nipples 3a, 3b of a sleeve shape instead of the joint fitting 3 of a ring shape are directly attached to the outer circumferential wall of the main pipe rail 1 by an irregular fitting screwing system, welding, etc. so as to be projected outward in the diametrical direction of the main pipe rail 1. A pressing seating face 2-3 formed in a connecting head portion 2-2 on the side of a branch pipe 2 is abutted on a pressure receiving seating face 1-3 on the side of the main pipe rail 1, and is engaged with this pressure receiving seating face 1-3, and is connected by fastening a nut 4 screwed into each of the above sleeve nipples 3a, 3b. 
However, in each of the above conventional common-rail injection systems, large stress is generated in a lower end inner peripheral portion P of the branch hole 1-2 by axial force applied to the pressure receiving seating face 1-3 by the internal pressure of the main pipe rail 1 and the pressing of the connecting head portion 2-2 of the branch connecting body such as the branch pipe 2. Therefore, a crack is easily caused with the lower end inner peripheral portion P as a starting point, and there is a possibility of generation of leakage of a fuel. The crack is next easily caused on the inner surface of the main pipe rail. This is because the main pipe rail is constructed by a thick cylinder, but a large tensile stress in the circumferential direction is caused on the inner surface since the main pipe rail has a large inside diameter.
The present invention is made in consideration of the above problems caused in the prior art, and an object of the present invention is to provide a common-rail injection system for a diesel engine in which the internal pressure fatigue strengths of the main pipe rail and the branch hole are raised by using transformation induced plastic type strength steel, and can be further improved by reducing the concentrating degree of stress generated in a crossing portion of the branch hole including the lower end inner peripheral portion with respect to the main pipe rail and a main pipe rail side flow passage.